Papers by Author: Didier Chaussende

Abstract: The thermochromic properties (color change with temperature) of n type doped SiC wafers of different polytypes (3C, 4H and 6H) have been investigated up to 500°C under air. It was found that 3C-SiC color passes from bright yellow at room temperature to deep orangeat 500°C leading to a color contrast (ΔE) as high as 64. The hexagonal polytypes undergo also a color change upon heating but far less pronounced, with ΔE values <20. All these semiconductors undergo band gap shrinkage upon heating which effect largely participated to the observed color change. This effect is very sensitive for 3C polytypesince its bandgap is already in the visible energy range at room temperature. The thermochromicity of 3C-SiC was found to be reversible thanks to its thermal stability and its resistance towards oxidation.

Abstract: 3C-SiC films have been grown on [100] n-doped Si substrates in a horizontal cold wall CVD reactor. Without the use of plasma enhancement, the precursors silane and propane are used to deposit silicon carbide films at T < 1200°C. The structure of the grown films has been investigated via FESEM, XRD and Raman spectroscopy. It has been found that the growth rates are between 200 and 300 nm/h. Additionally, structural analysis give evidence of polycrystalline phases. Reasons for that could be insufficient cracking of the precursors and homogenous nucleation of Si species in the gas phase.

Abstract: Because the well-known site-competition and step-controlled epitaxy rules cannot reasonably describe all the incorporation processes of the main impurities (Al and N) into 4H-SiC during epitaxy, the concept of replacement incorporation was proposed and applied to explain the experimental results published so far. In this model, the transient formation of C or Si vacancies at the surface or sub-surface of terraces is proposed to play a key role by destabilizing the impurities sitting on them. In addition to the availability of these vacancies at the surface, desorption was proposed to be a very important limiting process for Al incorporation while only occasionally relevant for N incorporation. The main 4H-SiC epitaxial growth parameters are reviewed and discussed according to the proposed replacement model.

Abstract: In this work, we have investigated the seeding stage of 4H-SiC top seeded solution growth through the systematic observation of surface morphologies and numerical simulation. Different growth temperatures, C- and Si-polarities, and different solvents have been studied. This is the first report of transient nucleation of parasitic 3C-SiC at the early stage of solution growth in pure Si solvent even at high temperature. This unusual phenomenon was attributed to a huge temperature difference between the seed crystal and solvent surface at the seeding stage. We demonstrated that preheating of the seed crystals or addition of Al were effective to prevent such parasitic 3C-SiC nucleation.

Abstract: Using a sessile drop method, investigation of the surface reconstruction of a Si-face, 4°off (0001) 4H-SiC surface in contact with pure silicon or Al-Si alloys has been carried out in the 1600-1800°C temperature range. In pure silicon and at 1600°C, the surface evolves with a two stage process: i) a fast step-bunching leading to parallel macrosteps and ii) a slower step leading equilibrium morphology, composed of (0001), (10-1n) and (01-1n) facets. Increasing the temperature to 1800°C or adding a few percents of aluminium drastically enhance the first stage, but strongly reduce the second one.

Abstract: Numerical simulation appeared till now as the only tool able to describe the SiC growth by PVT process, while the chemistry of the Si-C system and its coupling to mass transfer were not considered in a satisfactory way. To assess the chemistry of SiC crystal, the coupling of numerical and thermodynamic calculations computed by FEM, and by treating SiC as a solid solution, respectively, is presented. This enables the possibilities to control the activity of each component in SiC crystal during the growth. The link between growth conditions and SiC crystal chemistry could be one of the key issues to link the growth and the occurrence of cubic or hexagonal polytypes.

Abstract: We address the problem of nitrogen incorporation during bulk crystal growth of 4H-SiC and 6H-SiC by seeded sublimation method. The partial pressure of nitrogen and temperature dependence were considered in bulk SiC crystals. Free carrier concentration and incorporated nitrogen were determined using Raman spectroscopy and Secondary Ion Mass Spectrometry, respectively. The incorporated nitrogen at the (000-1) C-face of 4H-SiC and 6H-SiC is found to be independent of the polytype of the crystal. Higher desorption rate at Si-face compared to C-face is found, using a Langmuir equation, which is attributed to the difference in bond density between the two polar faces. The increased nitrogen desorption when growth temperature increases is believed to be the most contributing factor, based on the temperature dependent trends.

Abstract: The electronic quality of a Physical Vapour Transport (PVT) grown 15R-SiC crystal at different stages of growth was assessed by time-resolved optical pump-probe techniques. The measured differential transmittivity (DT) kinetics for the layers corresponding to the initial, middle and final stages of growth revealed clear differences in the decay of the DT signal, indicating a decreasing concentration of traps at the later stages of the crystal growth. The estimated trap concentration in the initial layer was N_T ≈ 10¹⁹ cm^-3, while it decreased down to less than 2×10¹⁸ cm^-3 in the top layer. The injection dependence of the diffusion coefficient at room temperature confirmed the gradual decrease of N_T in the layers corresponding to later stages of growth. Accordingly, the bipolar diffusion coefficient in the middle and the top layer was D_a ≈ 2 cm²/s, while D_a = 0.9 cm²/s was measured in the layer closest to the seed.

Abstract: We report on the stabilization of 15R Silicon Carbide (15R-SiC) grown by seeded sublimation method. It was found that polytype transitions are directly related to the occurrence of facets on the grown crystals. Once a foreign polytype is formed, its propagation is governed mainly by the interface shape of the crystal and its evolution during growth. A concave crystal shape enhances the expansion of foreign polytypes, usually formed at the periphery of the crystal. Then, foreign polytypes can either overlap the original polytypes (constant concave crystal shape) or form inclusions (change to convex crystal shape). On the contrary, an initially slightly convex interface repels foreign polytypes towards the edge of the crystal. The optimization of the growth interface shape can be a key issue towards the stabilization of bulk 15R-SiC crystals.

Abstract: Investigation of excess carrier dynamics in a 15R-SiC bulk layer grown by physical vapour transport (PVT) on 15R-SiC substrate has been carried out using pump-probe techniques: an interband carrier injection by a picosecond laser pulse and measuring the induced absorption and diffraction of a probe beam. For this task, differential transmittivity (DT) and light induced transient grating (LITG) techniques were used. Room temperature carrier lifetime varied in the 3 ns 8 ns range at excess carrier densities above ΔN₀ = 7×10¹⁷ cm^-3 and was ascribed to the recovery time of optically recharged carrier traps, and their activation energy of E_a = 75 meV was determined. The presence of recharged traps caused the injection-dependence of the diffusion coefficient D, whereby its value dropped below 0.1 cm²/s at ΔN₀ < 1×10¹⁸ cm^-3 and gradually increased up to 0.7 cm²/s at higher injections. At elevated temperatures (300 K < T < 700 K), when the traps are thermally activated, the diffusivity increased up to ~ 1.5 cm²/s and was independent on ΔN₀. The overgrown layer parameters were comparable to those of the used 15R PVT seed.